It is known to improve the corrosion resistance of high-temperature metal alloys, intended for internal components in combustion chambers and the like, by adding small amounts of some rare earth metal to the alloy. A disadvantage with this method is that also the strength could be influenced by the alloy additive. For land-based gas turbines and the like, it is common to provide high-strength alloys with protective surface layers. Swedish patent No. SE 7711357 describes how the corrosion resistance of a high-temperature alloy is increased by applying a surface layer by means of electrochemical deposition. The surface layer consists of a coherent matrix of Co or Ni with discrete particles containing some added substances from the group Cr, Si, Al, Ti, Ta, Be or a rear earth metal. Protective layers containing Al, Pt, Cr, Y, Sc, B and spraying of layers of Cr, Al with small additives of Y, Zr, Hf, or a rare earth metal, are also known in the art.
During combustion in a fluidized bed of coal of a varying quality and also other fuels, such as waste from the pulp industry or other waste, wood, peat, oil, brown coal, and the like, combustion gases and ash particles are formed which may have a corrosive and also erosive influence on the construction materials, primarily in the combustion chamber, the ash separator and the gas turbine. A corrosive effect from formed SO.sub.3 may lead to the protective surface layer of the metal being broken through by the formation of porous sulphides which are not capable of protecting the metal from further attack, which involves a risk of accelerated erosion attack.
In systems operating under potentially corrosive conditions, it is important to ensure that the construction materials have a good resistance to degradation caused by the attack of corrosion and erosion.